1. Field of the Invention
The present invention relates to modifying the surface of an object by contacting said surface with a liquid processing solution using the liquid applicator geometry and the Marangoni effect (surface tension gradients) to define and confine the dimensions of the wetted zone on said object surface. In particular, the invention relates to contouring or figuring the surface of an object using an etchant solution as the wetting fluid and using real-time metrology (e.g. interferometry) to control the placement and dwell time of this wetted zone locally on the surface of said object, thereby removing material from the surface of the object in a controlled manner. One demonstrated manifestation of this invention is in the deterministic optical figuring of thin glasses by wet chemical etching using a buffered hydrofluoric acid solution and the Marangoni effect.
2. Description of Related Art
Small-tool finishing or figuring of optical surfaces basically involves moving a small polishing tool in a controlled manner to shape the surface of an optic. It is a critical technology for producing optics for applications ranging from camera lenses for the consumer market, to large-aperture optics for inertial confinement fusion and space telescope systems. Examples of optics figured by these techniques include complex-figured aspheric lenses, continuous contour phase plates, Alvarez lenses, and optics requiring local figure correction after processing via traditional lap polishing. Small-tool finishing is simultaneously a mature technology and one undergoing continuous development driven by the high cost and reproducibility problems of processes when applied to ever-tightening figure tolerances.
Traditional techniques employ rotary polishing pads. Recent developments (e.g. U.S. Pat. No. 5,591,098 and references therein) utilize directed flow fields to impinge fine abrasive slurries onto the optical surface. Magnetorheological finishing (e.g. U.S. Pat. Nos. 5,499,313, 5,795,212, 5,839,944, 5,951,369) extend this concept by controlling the viscosity of specially formulated abrasive slurry by application of magnetic fields. Ion-beam milling techniques (e.g. U.S. Pat. Nos. 3,988,564, 5,969,368) are alternative, fundamentally different methods for high-accuracy optical figuring. All of the above-mentioned techniques suffer fundamental limitations. They rely on process knowledge of removal rates, and are therefore iterative processes: the workpiece must be dismounted from the machine and measured, reworked and remeasured, until specifications are met. Ion beam milling techniques require large, expensive vacuum processing chambers and are not applicable to all materials. Abrasive small-tool polishing techniques cannot be used to figure very thin optics since the local mechanical stresses involved cause workpiece deformations that impact removal control and can even cause breakage.
Material removal on optical surfaces can be accomplished by etching or dissolution methods (e.g. silicate glasses are soluble in hydrofluoric acid solutions), but until now, wet etching has not been employed to figure optics. The problem has been largely how to confine the wetted zone of etchant solution to a specific, stable geometry. A surface being etched is hydrophilic to the etching solution. If a bolus of etchant solution is moved along the surface of a workpiece, a thin liquid film will be left behind that will continue to etch the surface. Recently (U.S. Pat. No. 5,660,642), surface-tension gradient driven flow (the Marangoni effect) has been shown to be effective in causing this thin entrained film to flow off the surface of a workpiece back into the bulk liquid if said liquid is applied in the appropriate manner. The present invention utilizes the Marangoni effect to confine a wetted zone to a stable, defined geometry on the surface of a workpiece. The wetted zone remains of constant size and shape as it is moved around on the workpiece surface. This allows the fluid to act on the surface of the workpiece only in this wetted zone with no mechanical contact or induced mechanical stresses applied to the workpiece. This in turn allows for real-time local metrology of the processing, and allows for the processing of very thin 1 mm-thickness) plates that cannot be processed by other means.
It is an object of the present invention to provide a method and apparatus for modifying the surface of an object by contacting the surface of said object with a zone of liquid processing fluid such as an etching or deposition solution, and confining this zone by means of surface tension gradients (Marangoni effect). The size and shape of this wetted zone is defined by the mechanical design of the liquid applicator and the Marangoni effect. Additionally, this confined wetted zone can be, if desired, moved relative to the object surface in a controlled fashion so as to generate a pattern on the surface.
It is a further object of this invention to provide a method and apparatus for using real-time metrology and control algorithms to move this wetted zone in a deterministic fashion on the surface of the object for control of the figuring process.
It is a further object of this invention to produce large ( greater than 5 cm major axis) thin ( less than 1 mm thickness) glass sheets figured flat to optical tolerances by use of the above methods and apparatus.
A key feature of this invention is the establishment of a meniscus of an aqueous processing liquid on the surface of the object to be processed, simultaneous with a means to establish a surface tension gradient in the meniscus region, such that the surface tension of said liquid is weakest where the liquid attaches to the solid. The means to achieve this has been reduced to practice by issuing the liquid upward and out of a vertical tube placed in proximity to a downward facing surface, such that a circular liquid wetted zone is established on said surface and the liquid flows down the outside surface of the tube. The liquid in the meniscus near its zone of attachment to the solid surface is relatively quiescent, while the liquid in the meniscus region where it transitions to the free surface falling film flow is continuously refreshed with liquid being pumped out of the supply orifice (tube). A surface tension gradient can be established in this meniscus by introducing a volatile organic compound (voc) into the vicinity of this meniscus, if such voc has the property that it is at least slightly water-soluble and produces a large reduction in the surface tension of water when dissolved in minute concentrations. Ethanol and isopropanol are examples of such voc""s that are of relatively minor hazard.
Higher concentrations of this voc are absorbed into the meniscus zone adjacent to the solid surface than are absorbed in the vicinity of the continuously refreshed zone near the free surface flow. Thus, the surface tension of the liquid meniscus is lower near the solid surface than near the flowing free surface. This surface tension gradient is sufficiently strong to cause a flow in the meniscus region away from the solid surface toward the falling film. This flow prevents the wicking of the aqueous processing fluid onto a hydrophilic surface, and prevents a thin film from being deposited as the wetted zone is moved slowly relatively to the workpiece surface while it is attached to said surface.
The voc can be introduced into the vicinity of the meniscus by evaporation of a pool of the liquid phase or from a porous medium saturated with the liquid phase of the voc. Alternatively, it can be introduced by forced convection of a carrier gas, containing a small amount of the voc, into this vicinity. The voc absorbed into the aqueous processing fluid can be removed by absorption on an activated carbon filter medium, for example, or can be allowed to build up without decreasing effectiveness for long periods when a sufficiently large processing liquid reservoir is used.
Surface tension gradients can also be established by thermal gradients. Heating the workpiece, for example, can lower the surface tension of the liquid at the attached meniscus in the vicinity of the workpiece surface compared with the surface tension of the cooler fluid flowing at the free surface flow. The resultant surface tension gradient is equivalent to that chemically produced according to the above description, and will result in an equivalent flow.
Contact of the processing fluid (such as an etchant solution) in the manner described above does not require mechanical contact with the workpiece and does not generate any mechanical stresses in the workpiece. When applied to etching of polished glass or crystal surfaces for optical application, a backpressure need not be applied to the backside of the workpiece, and it can be left open. This allows for back-side monitoring of the workpiece by such means as interferometry, whereby the local workpiece thickness can be measured and the resultant data used for real-time control of the motion of the wetted zone relative to the workpiece to achieve a desired figure. Also, the lack of mechanical stress generated by this process enables the figuring to optical tolerances of very thin extruded glass sheets that are very difficult or impossible to figure by traditional techniques.
The object of this invention is not limited to figuring of optics by etching, but can be applied to processes in which deposition occurs onto a surface from an aqueous solution, such as epitaxial layer growth from solution onto substrates. It can also be used for local treatment of surfaces with aqueous solutions while keeping adjacent areas on the surface dry. One skilled in the art will find other uses for the general concept.